When submitting oils or other hydrocarbons to thermal cracking in a reactor several, major problems occur due to the production of coke during the process. When submitting oils or other hydrocarbons to thermal cracking in an indirectly fired rotating kiln there are also several major problems.
One such problem is keeping the coke, formed in the cracking reactions, from coating the reactor walls and internals thus impeding heat transfer from the heat source to the inside of the kiln. Often charges of sand or metal are added to the kiln to scrape the walls of the kiln as it rotates. Coke rarely deposits in a uniform layer. An uneven coke layer can result in hot spots and eventual failure of the kiln shell.
The second problem is getting the required heat from its source to the reaction site. Typically in a kiln, the heat transfer area in contact with the reactants is a small portion on the kiln shell. Further, charges added to the kiln without being previously heated outside of the kiln will form a resistance to heat transfer.
When the relatively cold oil or hydrocarbon feed is projected directly against the reactor shell, the resulting thermal shock can cause failure of the reactor shell.
In thermal cracking oils, the reaction temperature (and pressure) must be kept in a narrow operating range. If the temperature at the reaction site is too cold, the reaction will take longer and the feed rate will have to be reduced. If the temperature is too high, product quality and quantities are compromised. Therefore, for a given feedstock, reactor size and pressure, the temperature at the reaction site must be closely measured and controlled. This is difficult when the reactor wall cokes up or the metal charge has trapped the coke within it.
Finally, once the coke has been released, either when the reaction takes place or after it has been scraped off the surface it was attached to (i.e. on the charge or on the reactor walls), the coke must exit the reactor without plugging the exit from the reactor thereby causing pressure surges and failure of the reactor seals, often resulting in fires.
Rotating kilns, both directly fired (heat source or flame(s) inside the kiln) and indirectly fired (heat source or flame outside the kiln) have been used in various applications for more than 100 years. When hydrocarbons are being treated in a rotating kiln to make a specific slate of oil products, an indirectly fired kiln is used.
One of the earliest applications for indirectly fired kilns was the production of coal oil and gas by thermal cracking and vaporization of coal.
At present, no satisfying solution has been identified in response to the numerous technical difficulties encountered by the following prior equipment and/or processes.
Holighaus et al. (CA 1,221,047) mentions that to avoid coke deposits building up on the inside of the walls of the drum, the latter contained steel balls that remove deposits from the walls by attrition as the drum revolved. The kiln is slanted toward the exit end, where a stationary box is located. A screen, attached to the kiln, keeps the metal charge in the kiln. The box has two exits, one for the hydrocarbon vapours at the top and a pipe at the bottom of the box for the solids.
Bernt (CA 1,129,195) suggests that chains, attached to spoons, are effective in removing coke deposits from the walls of a rotating kiln.
Musha and Maeda (U.S. Pat. No. 4,014,643) describe a similar apparatus with chains attached to lifters to break down the coke on the kiln walls as the kiln rotates.
Klaus (CA 1,334,129) mentions that the solid pyrolysed coke is removed from the reactor walls by the grinding bodies and the resulting small particles are directed to the centre of the kiln with spiral fins and continuously removed from the reactor through ports in the reactor walls. The ports open into a stationary ring around the kiln. Vapours exit through the top of the ring, while the fine solids exit through the bottom of the ring. Screens keep the grinding bodies in the kiln.
Taciuk et al. (CA 2,186,658) describes a charge of ceramic balls or coarse granular solids provided within the vessel chamber. As the vessel rotates, the ceramic balls or the granular solids scour the vessel's internal surface and comminute the coke into fine solids. The coke is directed to one end of the kiln with spiral fins continuously welded to the reactor wall. A spiral chute with a screen at its entrance transports the coke up to the exit pipe. The exit pipe, at the centre of the exit end of the kiln, has a screw conveyor to take the coke out of the reactor.
These beds of solids constitute a resistance to heat transfer, especially when coke is captive in the interstices between the solids forming the charge.
Indirectly fired rotating kilns are not very efficient in transferring heat to the hydrocarbons to be cracked and/or vaporized through the shell. Some use a stream of solids circulating between two kilns: The process kiln, where the solids release the heat they contain to the hydrocarbon to be treated, and another kiln where the coke that deposited on the solids is burned off, heating the solids, which are then returned to the first kiln.
Taciuk et al., CA 1,120,418, suggest the use of a stream of sand to carry heat from an outer kiln, where the burner is situated, to the inner kiln, where the tar sands is vaporized and/or thermally cracked.
Raymond and McKenny, CA 2,151,792, suggest the use of a stream of ceramic or Pyrex® glass balls circulating between an indirectly fired rotating kiln where a coal and oil mixture are pyrolysed, and a directly fired kiln where the coke is burned off the balls, cleaning and heating them. The hot balls are then returned to the first kiln, where they release some of the heat required for the process.
In a similar process, Taylor, U.S. Pat. No. 5,423,891 mentions a heat carrying solid (HCS) such as iron oxide, aluminium oxide, refractory inert, fine mesh sand, or retorted residue from the starting waste material, circulating between a dryer, where the coke is burned off and the HCS is heated, and the thermal cracking kiln where the “gasification” of solid waste material takes place.
These prior art processes involve significant material handling difficulties encountered in the conveyance of large amounts of hot solids.
Others suggest the use of fins attached to the kiln walls in an effort to enhance heat transfer from the heat source through the reactor walls.
Peterson and Wilson, (CA 1,316,344), describe a plurality of fins extending from the inner wall and transmitting heat from the inner wall to the particulate material.
Kram et al. (U.S. Pat. No. 4,131,418) mention heat exchange fins on the inside of cooling tubes to enhance the cooling of solids particulates.
Hogan (U.S. Pat. No. 4,872,954) mentions fins affixed to the exterior surface of the drum of a retort for treating waste.
Fins continuously welded to the wall of a kiln can cause stress and failure of the kiln wall due to the differential expansions of the wall and of the fins. Also, fins inside the kilns are surfaces that are easily covered in coke causing hot and cool spots furthermore, they are difficult to clean.
Lifters and mixers in rotating kilns are mentioned in several patents, usually to enhance the mixing of material within a directly fired kiln (i.e. the flame is inside the kiln along with the material to be dried, burned, incinerated, calcined and/or decoked).
Tyler (U.S. Pat. No. 4,475,886), Leca (GB 1,534,302), Ellis (GB 2,150,271), Schoof (WO 1997/046843), Hojou (JP 2007 040615), Omiya (JP 2006 0309565) and Doeksen (CA 2315774) all describe lifters or mixers, attached to the kiln wall and protruding trough the ceramic lining of a directly fired kiln. Vering (U.S. Pat. No. 3,807,936) describes blade lifters to be used in kilns treating abrasive materials such as in cement clinkers.
Twyman (CA 1099507) mentions curved lifters, attached to the kiln wall, as mixing paddles in a directly heated kiln with flue gas as the source of heat. In a similar kiln, Musha et al. (U.S. Pat. No. 4,014,643) mentions attaching chains and spoons to the end of each mixing paddle to scrape the kiln walls and the lifter below clean of coke or other deposits in kilns used as dryers for slurries before they are fed to incinerators.
All these mixers and lifters are suggested as means to turn over the material to be treated and show more of the untreated material to the source of heat.
There was therefore a need for reactors allowing the thermal processing of various mixtures but free of at least one of the drawbacks of prior art known reactors and/or processes.
There was a further need for a process that addresses at least one of the problems of the prior art processes, and preferably all of them.
There was a further need for the recovering of valuable products and/or by-products during the process, and preferably for the recovery, in an environmental and acceptable way, of usable products and/or reusable by-products.
There was also a need for new uses for products recovered by thermal treatment.